Introduction

Full-arch implant-supported fixed restoration is an attractive treatment option due to the ability to provide immediate restorations, thereby reducing the duration of edentulism1. In 1998, Paulo Maló first proposed the all-on-4 treatment concept to support a full-arch restoration2. Numerous studies strongly support using 4 implants in the mandible and 6 in the maxilla for full-arch immediate restoration3,4,5. A minimum of 30 N-centimeters (Ncm) of initial stability is typically recommended for each implant in immediate full-arch restorations. Posterior cantilevers are generally not recommended in fixed prostheses because chewing forces can damage the implants and prostheses6,7,8. Immediate full-arch restorations in the maxilla pose challenges due to factors such as the proximity of the maxillary sinus, significant atrophy, and softening of the alveolar bone. Currently, there is no consensus regarding the use of four maxillary implants for immediate restoration, as the poorer bone quality in the maxilla compared to the mandible hampers the ability to achieve sufficient initial stability.

Zygomatic implants have been utilized to treat severely atrophic maxillas. While the success rate of zygomatic implants is impressive, the complexity of the procedure and the potential for serious complications restrict their widespread use.

9,10. Pterygoid implants can eliminate the posterior cantilevers of the prostheses; however, the surgical procedure is challenging due to limited access and visibility in the pterygoid region, and patient hygiene is more difficult to maintain11,12. Simultaneous or staged placement of implants during lateral sinus floor elevation (LSFE) is often used to address insufficient bone in the posterior maxilla13. In such cases, immediate restoration with implants placed simultaneously during sinus floor elevation is infeasible due to inadequate initial stability. A clinical decision should consider 3 fundamental elements: the current health status of the patient, the objective and preference of the patient, and the scientific evidence on the safety and efficacy of the surgical technique14. Generally, patients choose minimally invasive, quicker, and cost-effective approaches.

In the present study, we propose the immediate restoration of the short arch using 4 implants in the maxillary anterior region. This approach aims to enhance patients’ aesthetics and partial chewing function while allowing adequate healing time for both the bone grafting material and the implants in the maxillary sinus. Compared to zygomatic and pterygoid implants, implants placed after LSFE provide superior biomechanical advantages, create a better-contoured prosthesis, and are easier to maintain. By utilizing immediate restoration with 4 implants in the anterior region, we can effectively preserve the patient’s essential aesthetics and masticatory function.

Materials and methods

This is a retrospective study. Patients with maxillary edentulism and insufficient posterior bone volume receiving an immediate short arch restoration with 4 implants in the anterior region from July 2020 to December 2022 were enrolled. The inclusion criteria were as follows: (a) preoperative crosssectional CBCT images show sinus floor defects, (b) the anterior region had sufficient bone volume for the placement of 4 implants with adequate initial stability. (c) completed LSFE followed by dental implant placement, (d) well-documented medical charts reporting details regarding prosthetic complications. The exclusion criteria were as follows: (a) incomplete or low-quality clinical and radiographic documentation, (b) refusal to undergo the LSFE procedure; (c) untreated periapical disease or periodontal disease; (d) acute and chronic inflammation in the maxillary sinus; (e) uncontrolled diabetes and other systemic conditions; (f) pregnancy or lactation; (g) severe alcoholism/drug abuse; (h) smokers; (i) Type IV bone quality (Lekholm and Zarb classification). All patients provided informed consent before inclusion into the study. All surgeries were conducted at the Department of Oral Implantology, Tianjin Stomatological Hospital. The study protocol adhered to the tenets of the 2008 Helsinki Declaration and was approved by the Ethics Committee of Tianjin Stomatological Hospital (approval number: PH2023-B-017).

Fig. 1
Fig. 1
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Preoperative design. (a) diagnostic denture (b) Preoperative cone beam computed tomography (CBCT) scan (c) Prosthetically guided implant design.

An alginate impression was taken to create a diagnostic model that captured the patient’s centric occlusion relationship using a wax bite. A diagnostic denture was then fabricated, featuring randomly distributed radiopaque points across its surface (Fig. 1a). Following this, a cone beam computed tomography (CBCT) scan was conducted (Fig. 1b). Separate digital scans of both the patient’s diagnostic model and the denture were obtained. The digital scan data were integrated with the CBCT data using CoDiagnostiX Version 9.11 software (Straumann, Switzerland) to generate a comprehensive digital model encompassing the bone structure, soft tissues, and virtual prosthesis.

To facilitate prosthetically guided implant placement, implants were accurately designed, and digital guides were created (Fig. 1c). Two implants were planned for zones 1, 2, and 3. Additionally, an immediate short arch restoration with four implants was designed for zones 1 and 2, as there was insufficient bone volume in zone 3 to achieve initial stability during implant placement15.

Fig. 2
Fig. 2
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Surgical procedure. (a) Verification of guide accuracy. (b) Fixation of guide. (c) Implants placement. (d, e) Window openings in the lateral wall of the maxillary sinus. (f) tension-free wound closure.

The accuracy and reliability of the digital guides were verified through O-Bite (DMG, Germany), and the Template Fixation Pin (Institut Straumann AG, Switzerland) were inserted under local anesthesia (Fig. 2a, b). Implant cavities were prepared using a 2 mm pilot drill following the digital guide in the maxillary anterior region. After the removal of a digital guide, an incision was made along the alveolar ridge to expose the alveolar bone and the lateral wall of the maxillary sinus. Implant cavities were prepared by gradually transitioning to larger diameter drills, followed by the insertion of the implants (Fig. 2c). The implants used in this study were Straumann BLT (Straumann, Switzerland) and Nobel Active (Nobel Biocare, Sweden), both achieving initial stability exceeding 30 Ncm. Screw-retained abutments and healing caps were then installed, and bone augmentation was carried out at sites where the buccal bone plate was deficient. The LSFE procedure was performed in zone 3, beginning with the creation of a bone window using ultrasonic instruments. Subsequently, the maxillary sinus membrane was elevated, and Bio-Oss (Geistlich, Switzerland) was inserted (Fig. 2d, e). Simultaneous or delayed (8 months later) implant placement was performed, followed by tension-free wound closure (Fig. 2f). Postoperative CBCT was conducted to assess the bone level around the implants.

Fig. 3
Fig. 3
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Immediate restoration. a, Impression with splint. b, Immediate prosthesis. c, Completion of immediate restoration.

An immediate impression was obtained using a stabilization splint, and the occlusal relationship following surgery was recorded (Fig. 3a). Subsequently, computer-aided design/computer-aided manufacturing (CAD/CAM) resin prostheses were fabricated (Fig. 3b). The prosthetic length was determined based on the distal implant location to prevent cantilever and restore 6 to 8 teeth. The immediate restoration was completed within one week postoperatively (Fig. 3c). To ensure proper placement of the prosthesis, panoramic imaging was performed (Fig. 4). Patients were advised to consume soft foods and utilize an irrigator and interproximal brushes to maintain oral hygiene.

Fig. 4
Fig. 4
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Panoramic imaging of immediate restoration.

During the second stage of surgery, the screw-retained abutments were replaced after the implants in zone 3 achieved osseointegration. The immediate short arch restoration was removed, and scan bodies were connected to the implants to capture the soft tissue contours using TRIOS 4 (3Shape A/S, Denmark) (Fig. 5a, b). The scan bodies were then replaced and scanned with ICam4D (Imetric4D Imaging Sàrl, Switzerland) to determine the three-dimensional positions of the implants (Fig. 5c, d). A digital model capturing the occlusal relationship was created by intraoral scanning the immediate prosthesis with the oral scanner. Digital models of the soft tissue contours, implant positions, and occlusal relationships were aligned, leading to the design and fabrication of the final prostheses, which were completed and placed (Fig. 5e). Panoramic imaging was again performed to verify the proper positioning of the prosthesis (Fig. 5f). Finally, a postoperative CBCT was conducted to evaluate the bone levels surrounding the implants.

The occurrence of prosthetic complications and patient complaints of discomfort were recorded, and marginal bone loss of the implants used for immediate restoration was measured. Marginal bone loss was defined as the difference in proximal and distal mesial bone height of each implant, measured immediately post-operation and at the time of final restoration. To ensure objectivity, all measurements were performed independently by XB. L and HZ. D. Inter-rater reliability was assessed using Cohen’s kappa statistic, which showed almost perfect agreement (κ = 0.87).

Fig. 5
Fig. 5
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Final restoration. (a, b) Intraoral scanning for soft tissue contour. (c, d) Intraoral scanning for the position of implants. (e) Picture of final restoration in mouth. (f) Panoramic imaging of final restoration.

Statistical analysis was performed using SPSS software (IBM SPSS Statistics, v21.0; IBM Corp). For continuous variables (e.g., bone loss): Data normality was assessed using the Shapiro-Wilk test. Normally distributed data are presented as standard descriptive statistics (mean, standard deviation, minimum, maximum, and 95% CI). Non-normal data are reported as median (IQR) and analyzed with Wilcoxon signed-rank tests (paired) or Mann-Whitney U tests (unpaired).

Results

Ten patients participated in the study, consisting of 7 females and 3 males, with ages ranging from 39 to 66 years. Among them, 7 patients retained their mandibular teeth, while 3 received implant-supported full arch restorations. The short arches were restored to the bilateral cuspids in 4 patients and to the bilateral first premolars in 6 patients. The duration for replacing temporary short-arch prostheses varied from 8 to 21 months, with a mean of 13.7 ± 4.6 months. Notably, none of the 40 implants used for immediate restorations in any of the patients failed.

In terms of prosthetic complications, five patients experienced fractures of the short-arch prostheses between 5 and 16 months following immediate restoration (see Table 1). All fractures occurred specifically within the distal cantilever region of the interim CAD/CAM resin prostheses, primarily at the connector or pontic areas. It is important to highlight that these interim prostheses were monolithic resin restorations without metal framework reinforcement. Further analysis revealed no direct correlation between prosthesis fractures and specific implant positions; instead, the failures appeared to be more closely related to the cantilever design, the mechanical limitations of the resin material, and individual variations in occlusal loading.

Table 1 Demographic and surgical data of participants.
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Meanwhile, 2, 3, and 2 patients complained of impacted articulation, cheek/lip biting, and poor mastication, respectively. The average marginal bone loss of implants for immediate restorations was 0.61 mm (95% CI 0.50 to 0.72 mm) (Table 2).

Table 2 Marginal bone loss of implants.
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Discussion

The findings of this study demonstrate the feasibility of immediate short arch restoration using four implants in cases of maxillary edentulism. This is supported by the absence of implant failure, minimal marginal bone loss, and manageable prosthetic complications.

Immediate prosthetic restoration achieves more midfacial soft tissue levels and bone volume maintenance compared to delayed prosthetic restoration16. Immediate fixed bridge restorations with implants are significantly more complicated in the edentulous maxilla than in the mandible. The bone density of the maxilla is usually lower than that of the mandible, which complicates the achievement of adequate initial stability during implant placement17. Achieving adequate initial stability with zygomatic implants is a highly technical process. Lateral sinus floor elevation (LSFE) is a conventional method used to address insufficient bone height in the posterior region, known for its reduced complexity and proven reliability. However, one of its drawbacks is the lengthy treatment duration, which can be challenging for edentulous patients.

In this case, an edentulous maxilla with insufficient bone height in the posterior region required LSFE. To facilitate an immediate short arch restoration, four implants were placed in the anterior region. This approach made use of the bone located between the bilateral maxillary sinuses to ensure initial stability. An immediate restoration was performed, providing both aesthetic benefits and functional masticatory capability while the patient awaited osseointegration of the implants in the maxillary sinus. The implant survival rate for short arch immediate restorations was 100%, with no significant complications, demonstrating that this method is a viable option.

Micromotion of the implant-bone interface during healing should be controlled to within 150 μm to achieve osseointegration18. Linking implants to an immediate restoration helps distribute stress and keeps the micromotion of each implant within acceptable limits. Clinical studies indicate that immediate restoration with splinted connections is effective when the initial stability exceeds 30 Ncm. The marginal bone loss observed in immediately loaded implants is similar to that in conventionally healed implants, with most loss occurring within the first 3 to 4 months post-placement. If stresses cause micromotion to exceed acceptable limits during the osseointegration process, significant marginal bone loss or even osseointegration failure may occur. While immediate restoration of a short arch that only restores the anterior region can lead to implant complications due to large lateral forces, the present study found no such complications.

Additionally, no implant failure was found, and the average marginal bone loss of implants for immediate restorations was 0.61 ± 0.34 mm at an average follow-up of 13.4 ± 4.5 months. The criterion for implant success is defined as marginal bone resorption controlled within 1.5 mm in the first year and less than 0.2 mm per year thereafter19. This positive outcome can be attributed to effective occlusal force management and a high level of patient compliance. However, the main complication associated with immediate restorations was the fracture of the temporary prosthesis, likely due to the limited strength of resin prostheses and insufficient support from the premolar. It is clear that not all patients are able to manage their occlusal forces within the tolerances required for short arch restorations.

In typical centric occlusion, the posterior teeth make uniform contact to maintain the vertical dimension under masticatory forces. However, during forward and lateral movements, the guidance provided by the anterior teeth causes the posterior teeth to disengage. The immediate short arch restoration in this study involved only 6 to 8 teeth, which was insufficient for adequate posterior support and necessitated continuous control of occlusal forces. Fractures of the restorations occurred 5 to 16 months after the immediate restoration, during which time the implants osseointegrated without any implant failures or significant marginal bone loss.

Prosthetic fractures are associated with material stress fatigue and excessive occlusal forces. In clinical practice, the risk of these fractures can be minimized by using reinforcing metal brackets in temporary prostheses, controlling occlusal forces, and preventing parafunctional movements. If restorations become fractured, they can either be bonded with a self-adhesive resin or refabricated as needed.

Prosthetic complications were noted in all three patients who received implant-supported full-arch restorations in their mandibles, compared to only two out of seven patients with natural teeth in their mandibles (Table 2). The lack of physiologic mobility in implants subjected the short arch restorations to greater occlusal forces, underscoring the need to reduce these forces in affected patients. The anticipated duration to achieve the final restoration was approximately 8 months for patients undergoing LSFE with simultaneous implant placement and about 12 months for those receiving staged implant placement. However, delays in follow-up appointments due to COVID-19 extended the overall treatment duration. Timely follow-up and effective management of occlusal forces may help reduce prosthetic complications.

Compared to implant failure and unacceptable marginal bone resorption, prosthetic complications (prosthesis fracture) appear to be more acceptable. Zygomatic implants are sometimes used in cases of insufficient bone volume in the maxillary posterior region, but their failure rate (24.24%) and complication incidence (53.73%) are significantly higher than those of implants placed after LSFE20. The common complications of zygomatic implants include peri-implantitis21 and maxillary sinus mucosal thickening22, which are more difficult to manage than prosthetic fractures. Although immediate short arch restoration, covering only 6 to 8 teeth, led to some complaints related to articulation, cheek bite, and poor mastication, most patients were satisfied with the treatment. Alternative strategies for immediate full-arch rehabilitation in severely atrophic jaws have been proposed, including nerve relocation with immediate restoration23 and the use of custom-made subperiosteal titanium implants24. Other less invasive techniques, such as a modified transalveolar two-step osteotome-mediated approach, have been reported with favorable mid-term outcomes25. Compared with other techniques, the LSFE protocol selected for this study was tailored to the specific bone defect dimensions and anatomical conditions of each patient. This approach aimed to ensure predictable grafting and facilitate immediate restoration.

When interpreting the results, it is important to consider the limitations of this research. The study was a small-sample, retrospective analysis without a control group, which limits statistical power and prevents definitive conclusions regarding the performance of this protocol in comparison to alternative methods. Although the mean follow-up period of approximately 14 months is adequate for evaluating short-term feasibility and safety, it is insufficient for assessing long-term outcomes. Therefore, the findings should be viewed as preliminary. Future studies with larger samples, multicenter involvement, prospective designs, and extended follow-up are needed to validate the long-term efficacy and stability of this approach.

Conclusion

In summary, the findings of this preliminary study indicate the potential feasibility of immediate short arch restoration using 4 implants as a treatment alternative for selected patients with maxillary edentulism and posterior bone deficiency requiring LSFE. The observed marginal bone loss was acceptable, and complications were manageable within the short follow-up period. However, these results are limited by the study’s retrospective design, small sample size, and lack of a control group. Consequently, while the protocol appears viable, larger prospective studies with comparative groups and long-term follow-up are essential to confirm its clinical efficacy and mechanical stability.